It is known to one skilled in the art that a less unsaturated hydrocarbon compound can be produced by a thermal cracking process. For example, a fluid stream containing a saturated hydrocarbon such as, for example, ethane, propane, butane, pentane, naphtha, and the like and combinations thereof can be fed into a thermal (or pyrolytic) cracking furnace. Within the furnace, the saturated hydrocarbon is converted to a less unsaturated hydrocarbon compound such as, for example, ethylene or propylene. Such less unsaturated hydrocarbons are an important class of chemicals that find a variety of industrial uses. For example, ethylene can be used as a monomer or comonomer for producing a polyolefin. Other uses of unsaturated hydrocarbons are well known to one skilled in the art.
However, such less unsaturated hydrocarbon produced by a thermal cracking process generally contains an appreciable amount of less desirable highly unsaturated hydrocarbon(s) such as alkyne(s) or diolefin(s). For example, propylene produced by thermal cracking propane, natural gas liquids or other saturated hydrocarbons are generally contaminated with a highly unsaturated hydrocarbon, such as methyl acetylene. For commercial purposes, it is desirable to selectively hydrogenate the highly unsaturated hydrocarbons to a less unsaturated hydrocarbon, such as propylene, but not to a saturated hydrocarbon such as propane, in a hydrogenation reaction. Additionally, propylene produced by thermal cracking of propane, natural gas liquids or other saturated hydrocarbons may also be contaminated with a highly unsaturated hydrocarbon such as propadiene. Again, for commercial purposes, it is desirable to isomerize the propadiene to another highly unsaturated hydrocarbon, such as methyl acetylene, which may then be selectively hydrogenated to a less unsaturated hydrocarbon such as propylene, but not to a saturated hydrocarbon such as propane.
As an alternative example, ethylene produced by thermal cracking of ethane, natural gas liquids or other saturated hydrocarbons may be contaminated with a highly unsaturated hydrocarbon such as acetylene. For commercial purposes, it is desirable to selectively hydrogenate the highly unsaturated hydrocarbon to a less unsaturated hydrocarbon such as ethylene, but not to a saturated hydrocarbon such as ethane, in a hydrogenation reaction.
Catalysts comprising palladium and an inorganic support, such as alumina, are known catalysts for the hydrogenation of highly unsaturated hydrocarbons such as alkynes and/or diolefins. In the case of the selective hydrogenation of acetylene to ethylene, a palladium and silver catalyst supported on alumina can be employed. Such catalysts are disclosed in U.S. Pat. Nos. 4,404,124 and 4,484,015, the disclosures of which are incorporated herein by reference. The operating temperature for this hydrogenation process is selected to maximize hydrogenation of highly unsaturated hydrocarbon such as alkyne (e.g., acetylene) to its corresponding less unsaturated hydrocarbon such as alkene (e.g., ethylene) thereby removing the alkyne from the product stream while minimizing the amount of alkene which is hydrogenated to a saturated hydrocarbon such as alkane (e.g., ethane).
It is also generally known to those skilled in the art that impurities, such as carbon monoxide, and sulfur impurities, such hydrogen sulfide, carbonyl sulfide, carbon disulfide, mercaptans, thiophene, organic sulfides, organic disulfides, organic trisulfides, organic tetrasulfides and organic polysulfides, which are present in an alkyne-containing feed or product stream can poison and deactivate a palladium-containing catalyst. For example, carbon monoxide is well known to temporarily poison or inactivate such a hydrogenation catalyst. It is also generally known by those skilled in the art that a sulfur impurity such as a sulfur compound (such as H2S, COS, mercaptans, thiophene, organic sulfides, organic disulfides and organic polysulfides), when present during the hydrogenation of highly unsaturated hydrocarbons such as diolefins (also referred to as alkadienes) or alkynes to less unsaturated hydrocarbons such as monoolefins (also referred to as alkenes), can poison and deactivate hydrogenation catalysts. This is especially true in a depropanizer hydrogenation process because the feed stream from the depropanizer being sent to the acetylene removal unit (also referred to as “ARU”) of such depropanizer hydrogenation process typically contains low levels of a sulfur compound(s) with the possibility of transient spikes in the level of such sulfur compound(s). Thus, the development of a catalyst composition and its use in processes for the hydrogenation of highly unsaturated hydrocarbons such as diolefins or alkynes to less unsaturated hydrocarbons such as monoolefins in the presence of a sulfur impurity such as a sulfur compound would also be a significant contribution to the art and to the economy.
A palladium-containing “skin” catalyst in which palladium is distributed on the surface or “skin” of the catalyst has been developed which is known to be more selective and active than a non-skin catalyst in converting highly unsaturated hydrocarbons in an ethylene stream to less unsaturated hydrocarbons. Such a catalyst is disclosed, for example, in U.S. Pat. No. 4,484,015, the disclosure of which is incorporated herein by reference. It is known that the catalyst selectivity is determined, in part, by the skin thickness. Generally, catalyst selectivity decreases as the skin thickness increases. There is therefore an ever-increasing need to develop a catalyst having a better “skin” on the catalyst for a better selective hydrogenation of a highly unsaturated hydrocarbon, such as an alkyne, to a less unsaturated hydrocarbon, such as an alkene, without further hydrogenation to a saturated hydrocarbon, such as an alkane.
Methylacetylene and propadiene (collectively referred to as “MAPD”) are impurities present in the front-end depropanizer ARU stream in ethylene plants. They are usually removed by selective hydrogenation. Otherwise, they are concentrated in the propylene/propane stream and must be removed by fractionation or in a secondary hydrogenation process. Excessive propylene loss occurs during fractionation when the MAPD level is high.
Hydrogenation of methylacetylene proceeds more readily than propadiene because, in many ways, propadiene behaves like propylene. The conversion of the propadiene can be increased if the propadiene is first isomerized to methylacetylene. In front-end depropanizer ARU service, palladium without promoters usually provides a good conversion of the propadiene because the palladium catalyzes the hydroisomerization of the propadiene to methylacetylene. However, it is well known that palladium catalysts without promoters are poor selective hydrogenation catalysts as far as in converting acetylene to ethylene. The selectivity is improved by the addition of silver to the palladium but the silver also drastically cuts the hydroisomerization activity of the palladium, thus greatly reducing the conversion of the propadiene.
There remains a need therefore, for a highly selective and active hydrogenation catalyst useful, for example, in front-end depropanizer ARU service. There further remains a need for a hydrogenation catalyst converting highly unsaturated hydrocarbons, such as methylacetylene into less unsaturated hydrocarbons, such as propylene. There still further remains a need for a catalyst for the isomerization of highly unsaturated hydrocarbons, such as propadiene, to another highly unsaturated hydrocarbon, such as methyl acetylene, such that the other highly unsaturated hydrocarbon may then be hydrogenated to a less unsaturated hydrocarbon. There still further remains a need for such a catalyst which does not cause significant production of saturated hydrocarbons, such as ethane.